After the Bolshevik Revolution, a mainly Russian and Armenian grouping of Baku Bolsheviks declared a Marxist republic in Azerbaijan. Muslim nationalists separately declared the establishment of the Azerbaijan People's Democratic Republic in May 1918 and formed the "Army of Islam," with substantial help from the Ottoman Turkish army, to defeat the Bolsheviks in Baku. The Army of Islam marched into the capital in September 1918, meeting little resistance from the Bolshevik forces. After some violence against Armenians still residing in the city, the new Azerbaijani government, dominated by the Musavat, moved into its capital. Azerbaijan was occupied by Ottoman Turkish troops until the end of World War I in November 1918. British forces then replaced the defeated Turks and remained in Azerbaijan for most of that country's brief period of independence.

Facing imminent subjugation by the Red Army, Azerbaijan attempted to negotiate a union with Persia, but this effort was mooted when the Red Army invaded Azerbaijan in April 1920. Russian leader Vladimir I. Lenin justified the invasion because of the importance of the Baku region's oil to the Bolsheviks, who were still embroiled in a civil war. The Red Army met little resistance from Azerbaijani forces because the Azerbaijanis were heavily involved in suppressing separatism among the Armenians that formed a majority in the Nagorno-Karabakh area of southcentral Azerbaijan. In September 1920, Azerbaijan signed a treaty with Russia unifying its military forces, economy, and foreign trade with those of Russia, although the fiction of Azerbaijani political independence was maintained.

Data as of March 1994